1. Field of the Invention
This invention relates to apparatus for utilizing solar radiation and in particular to an apparatus for absorbing solar radiation for use in heating fluids to elevated temperatures.
2. Description of the Prior Art
It is known to collect solar radiation using a wide variety of designs. A solar water heater using a conical-shaped coil of metal tubing enclosed within a double walled, dome-shaped cover is disclosed in U.S. Pat. No. 2,213,894. The coil is self-supporting and does not contain a backing member. U.S. Pat. No. 2,402,326 discloses a solar heater which uses a frusto-conical-shaped support containing a spiral grooveway on its upper surface. Disposed in the grooves is a metal tube. Sunlight which passes through the protective glass cover is absorbed, heating the mass of the frusto-conical support and enclosed tube. In U.S. Pat. No. 3,853,114 a solar heater is disclosed which uses a double walled transparent box. The cavity of the inner box is filled with a particulate, heat absorbing material. Disposed within the inner cavity and surrounded by the particulate material is a helical or sinuous array of tubing containing the fluid to be heated. As the particulate matter absorbs solar energy, the entire mass of the inner cavity becomes heated.
The solar collector which has found the widest application is the flat-plate collector. Basically, this type of collector is simply an absorbing surface covered by a transparent cover to trap heat within the collector and reduce convective radiation loss. The absorbed radiation is removed from the absorbing surface by means of a working fluid which is generally contained in metal tubing. This design has the advantage of low fabrication, installation and maintenance costs. These collectors are usually mounted in a fixed position with a south to southwest orientation and are therefore useful only during the middle portion of the day. In order to increase the efficiency of flat-plate collectors, various sun-tracking and panel reflectors have been proposed. These embodiments, however, add considerably to the cost and maintenance of the collector. Moreover, the typical flat-plate collector panel is relatively large (4 ft by 8 ft) and involves considerable weight.